L-cysteine, an amino acid playing an important role in sulfur metabolism in all living organisms, is used not only in the synthesis of biological proteins such as hair keratin, glutathione, biotin, methionine, and other sulfur-containing metabolites, but also as a precursor for biosynthesis of coenzyme A.
Known methods of producing L-cysteine using microorganisms include: 1) a method of biologically converting D,L-2-aminothiazoline-4-carboxylic acid (D,L-ATC) to L-cysteine using microorganisms, 2) a method of producing L-cysteine by direct fermentation using E. coli (EP0885962B; Wada M and Takagi H, Appl. Microbiol. Biochem., 73:48-54, 2006), and 3) a method of producing O-phosphoserine by fermentation using microorganisms, and converting O-phosphoserine into L-cysteine by reacting O-phosphoserine with a sulfide under the catalytic action of O-phosphoserine sulfhydrylase (Korean Patent No. 1381048). In particular, for the production of cysteine by the method 3) at high yield, the precursor, O-phosphoserine, should be produced in excessive amounts.
In this regard, the present inventors have made extensive efforts to discover an appropriate export factor that can smoothly export O-phosphoserine produced in an O-phosphoserine-producing microorganism from cells. Specifically, the present inventors have discovered an RhtB variant as a protein having O-phosphoserine-exporting activity (Korean Patent Application Publication No. 10-2014-0133751) and a novel O-phosphoserine-exporter (Korean Patent Application Publication No. 10-2014-0133754), and have confirmed that O-phosphoserine concentration increased when these proteins were activated in an O-phosphoserine-producing microorganism.